1. Field of the Invention
The present invention relates to a digital broadcast receiving tuner and to a digital broadcast receiving device incorporating such a tuner.
2. Description of the Prior Art
Normally, a digital broadcast receiving tuner has necessarily been incorporated in any of the conventional receiving devices for receiving a variety of digital broadcast signals transmitted from any of the communication broadcast satellite stations, any of the digital signal broadcast satellite stations, and any of the conventional digital ground wave broadcast stations.
FIG. 5 exemplifies a typical constitution of a conventional tuner for receiving a digital broadcast signal. The tuner shown in FIG. 5 consists of a conventional digital satellite broadcast receiving tuner capable of down-converting the received digital signal based on the direct conversion format. The conventional digital broadcast receiving tuner shown in FIG. 5 incorporates the following: an RF (radio frequency) input terminal 1, an RF amplifier 2, an RF attenuator 3, a variable gain amplifier 4, mixers 5 and 6, amplifiers 7 and 8, a 90° phase shifter 9, a voltage-controlled oscillator 10, a PLL (phase-locked loop) circuit 11, and output terminals 12 and 13. Of these, normally, the variable gain amplifier 4, the mixers 5 and 6, the amplifiers 7 and 8, and the 90° phase shifter 9, are jointly loaded in a chip consisting of a direct conversion IC (integrated circuit).
Initially, an RF signal transmitted from a satellite broadcast station is received by an antenna (not shown) on the ground. Next, an RF signal in the 12 GHz frequency band received by the antenna is initially amplified up to a proper level and then down-converted into an RF signal in the 1 GHz frequency band by a low noise block down converter (LNB) (not shown), and finally, the converted RF signal is transmitted to the RF input terminal 1 installed in the digital broadcast receiving tuner shown in FIG. 5. Then, the RF signal input to the RF input terminal 1 is amplified by the RF amplifier 2. Actual level of the amplified RF signal is then properly adjusted by the RF attenuator 3 and the variable gain amplifier 4. The level-adjusted RF signal is then divided into two part and then transmitted to the mixers 5 and 6. Note that the gains of the RF attenuator 3 and the variable gain amplifier 4 are variable by an AGC (automatic gain control) controlling voltage supplied from an external source comprising a demodulating IC.
Either of the RF signals split into two parts is then mixed with a locally oscillating signal generated from the voltage-controlled oscillator 10 by means of the mixer 5, and is thereby converted into an I baseband signal, which is then transmitted to the output terminal 12 via the amplifier 7. The 90° phase shifter 9 shifts the phase of the locally oscillating signal generated from the voltage-controlled oscillator 10 by 90 degrees. The other RF signal split into two parts is initially mixed with a locally oscillating signal output from the 90° phase shifter by means of the mixer 6, and is thereby converted into a Q baseband signal before being transmitted to the output terminal 13 via the amplifier 8. Based on the serial operations performed as described above, the I baseband signal output from the output terminal 12 and the Q baseband signal output from the output terminal 13 are individually provided with own phase being different from each other by 90 degrees. Note that the PLL circuit 11 properly controls the voltage-controlled oscillator 10 in order that the locally oscillating signal output from the voltage-controlled oscillator 10 can correctly become a specific frequency substantially identical to that of the RF signals split into two parts.
Next, another typical example of a conventional digital broadcast receiving tuner is exemplified in FIG. 6. The conventional digital broadcast receiving tuner shown in FIG. 6 consists of a digital ground wave receiving tuner capable of down-converting the received signal by double conversion, comprising the following: an RF input terminal 21, an RF amplifier 22, an RF attenuator 23, a mixer 24, a voltage-controlled oscillator 25, a PLL circuit 26, a variable gain amplifier 27, another mixer 28, a local oscillator 29, and an output terminal 30. The mixer 24 as the first down-converting IC and the other mixer 28 as the second down-converting IC, are discretely loaded in an individual semiconductor chip. Normally, the variable gain amplifier 27 is included in either of the first and second down-converting ICs. In place of the local oscillator 29 oscillating a stationary frequency, it is also allowable to provide another voltage-controlled oscillator for oscillating an optional frequency and another PLL circuit for controlling operation of the newly provided voltage-controlled oscillator.
The RF signal received by a ground antenna (not shown) is transmitted to the RF input terminal 21 installed in the conventional digital broadcast receiving tuner shown in FIG. 6.
The RF signal transmitted to the RF input terminal 21 is then amplified by the RF amplifier 22, and then, actual level of the amplified RF signal is properly adjusted by the RF attenuator 23 before being transmitted to the mixer 24.
The RF signal output from the RF attenuator 23 is mixed with a locally oscillating signal output from the voltage-controlled oscillator 25 by means of the mixer 24, and is thereby converted into an intermediate frequency signal, and then, the actual level of this intermediate frequency signal is properly adjusted by the variable gain amplifier 27 before being transmitted to the other mixer 28. Note that the gains of the RF attenuator 23 and the variable gain amplifier 27 are individually subject to variation in correspondence with an AGC controlling voltage supplied from an external source comprising a demodulating IC. Further, the PLL circuit 26 properly controls operation of the voltage-controlled oscillator 25 in order that the locally oscillating signal output from the voltage-controlled oscillator 25 can correctly become a specific frequency so that the difference between the locally oscillating signal and the RF signal frequency can exactly coincide with the actual frequency of the intermediate frequency signal.
Next, the intermediate frequency signal output from the variable gain amplifier 27 is mixed with a locally oscillating signal output from the local oscillator 29 by means of the mixer 28 so as to be converted into a baseband signal, which is then transmitted to the output terminal 30.
Nevertheless, whenever operating either of the above-referred conventional digital broadcast receiving tuners exemplified in FIGS. 5 and 6, there has been many instances in which optimal performance characteristics could hardly be secured depending on the actual condition on the way of receiving RF signals. For example, as shown in FIG. 7A, insofar as the actual level of an RF signal within a receivable frequency band width remains flat, the carrier-to-noise (C/N) characteristics should closely be scrutinized. Conversely, when the actual level of an RF signal within a receivable frequency band width is not flat, distortion characteristics should closely be scrutinized. Inconveniently, however, either of the above-referred digital broadcast receiving tuners shown in FIGS. 5 and 6 respectively perform identical operations even when the actual level of the received signal within a receivable frequency band width remains flat or irregular, and thus it has thus far been quite difficult to constantly secure optimal performance characteristics at least in either of the cases in which the actual level of the received signal within a receivable frequency band width remains flat or being irregular.